For the benefit of others who arrive here from Google or other forum users, I was able to get my Dell UP2715K to work with my 6700K + 1080Ti system running High Sierra 10.13.6 by using Lilu + WhateverGreen. I'm using iMac17,1 SMBIOS.
Add these boot flags:
agdpmod=pikera nv_spanmodepolicy=1
Previous users have used Lilu + NvidiaGraphicsFixup, but NvidiaGraphicsFixup has been superseded by WhateverGreen.
Also, don't use the old/deprecated flag ngfxpatch=pikera
I love using this screen! I bought it second-hand from eBay for $600 AUD delivered from the US ($435 USD)